A next-generation mobile communication system requires a high data transmission rate and reliable transmission. Accordingly, a robust channel coding method is required. Since a turbo code has a structure in which two recursive convolutional codes are concatenated in parallel with an interleaver interposed therebetween and has excellent performance because a channel capacity is close to Shannon capacity, the turbo code is called a channel code suitable for the next-generation mobile communication system.
FIG. 1 is a block diagram showing the configuration of a general apparatus for encoding a turbo code.
A first constituent encoder 1 receives and encodes information bits and generates first redundant bits and a second constituent encoder 3 receives and encodes the information bits rearranged by an encoder interleaver 2 and generates second redundant bits. In the turbo encoding, the first or second redundant bits are parity bits.
For highly reliable decoding of the apparatus for decoding the turbo code, the constituent encoders of the apparatus for encoding the turbo code employs a method of inserting a tail bit and forcedly ending trellis or a circular coding method. In the circular coding method, since an initial state and a final state of the trellis are equally set and an additional bit is not inserted, band efficiency is higher than the method of inserting the tail bit. Here, the initial state and the final state are called a circular state. In order to decide the circular state, the initial state is set to a zero state, encoding is performed, and the final state is obtained. The circular state can be calculated using the final state and a specific encoder structure. In addition, the calculated circular state is set to the initial state and encoding is performed again, thereby generating the redundant bits.
FIG. 2 is a block diagram showing the configuration of a general apparatus for decoding a turbo code.
The information bit sequence and the first and second redundant bit sequences output from the apparatus of FIG. 1 are modulated and the modulated bit sequences are transmitted to a receiver having the apparatus for decoding the turbo code of FIG. 2. The receiver receives and demodulates the modulated bit sequences and supplies the demodulated bit sequences to the apparatus for decoding the turbo code of FIG. 2.
A first constituent decoder 4 receives and decodes the demodulated information bit sequence and the demodulated first redundant bit sequence and computes extrinsic information of the first constituent decoder 4. A decoder interleaver 5′ rearranges the extrinsic information of the first constituent decoder 4 and outputs the rearranged extrinsic information to a second constituent decoder 6. The second constituent decoder 6 receives and decodes the demodulated information bit sequence, the demodulated second redundant bit sequence and the extrinsic information of the first constituent decoder rearranged by the decoder interleaver 5 and computes extrinsic information of the second constituent decoder 6. A decoder deinterleaver 7 rearranges the extrinsic information of the second constituent decoder 6 and outputs the rearranged extrinsic information to the first constituent decoder 4. The above-described process corresponds to a first iterative decoding process. Such an iterative decoding process is performed until desired decoding performance is obtained.
The apparatus for decoding the turbo code includes a plurality of constituent decoders. The constituent decoders compute various types of metrics and perform a decoding operation. Such metrics include transition metrics, forward metrics and backward metrics. A log likelihood ratio (LLR) of the information bits is calculated on the basis of the forward and backward metrics.
Meanwhile, the HARQ is known well as a link control protocol for requesting re-transmission of a packet to a transmitter in the case where an error occurs in the packet received by the receiver.
The packet re-transmission operation using the HARQ is divided into a rate matching process performed by the transmitter and a de-rate matching process performed by the receiver. In particular, a process of converting a codeword packet encoded at a mother code rate into a sub-packet having a size to be transmitted by the transmitter is called sub-packet generation or rate matching (RM).
An example of the rate matching includes circular buffer based rate matching (CBRM). In order to generate the sub-packet using the CBRM, first, a codeword packet having a mother code rate is stored in a bitwise circular buffer (CB). Here, the CB indicates a buffer having circular characteristics in which, when the buffer is accessed, a first bit index of the buffer is accessed after a last bit index of the buffer is accessed. The sub-packets necessary for transmission are consecutive indexes of the buffer and are generated by retrieving a portion of the codeword packet stored in the circular buffer. At this time, in the CB, an index of a buffer in which the sub-packet is started is called a starting position and an index of a buffer in which the sub-packet is ended is called an ending position.
Several sub-packets used for initial transmission and re-transmission using the HARQ method are generated from one codeword packet. The several sub-packets generated at that time can be identified by the length of the sub-packets and the starting positions of the sub-packets. Each of the sub-packets which can be identified is called a redundancy version (RV) and RV information indicates the agreed starting position of the RV.
In each HARQ transmission, the transmitter Tx transmits the sub-packets via a data channel and transmits the RV information of the generated sub-packets via a control channel. The receiver Rx maps the sub-packets received via the data channel to the accurate position of the codeword packet using the RV information received via the control channel.
The circular buffer is divided into a physical circular buffer and a virtual circular buffer according to implementation methods.
First, in the case where the physical circular buffer is used, when the codeword packet which is forward error correction (FEC)-encoded at the mother code rate is stored in the CB, systematic bits and parity bits of the codeword packet are interleaved on the sub-block level, the interleaved systematic bits are first stored in the CB, and the interleaved parity bits are then stored in the CB. At this time, filler bits inserted into the codeword packet and dummy bits inserted for interleaving may be removed. If the filler bits and the dummy bits are not removed, the filler bits and the dummy bits may be removed in the process of generating the sub-packets, if necessary.
In the virtual circular buffer, a memory having a matrix shape, which is used for interleaving the systematic bits and the parity bits, is used instead of using an additional circular buffer memory. First, the codeword packet which is FEC-encoded at the mother code rate is stored in an interleaver matrix in order to divide the codeword into the systematic bits and the parity bits and interleave the systematic bits and the parity bits. When the sub-packets are generated from the stored codeword packet, the memory is accessed in consideration of an interleaving pattern.
Hereinafter, a fixed or adaptive RV of the CBRM using the physical circular buffer will be described.
The sub-packets necessary for all the HARQ transmission may be generated by selecting one of V starting position candidates. Accordingly, the RV information indicates one of V agreed starting positions regardless of the number of times of HARQ transmission.
A method of defining the RV information indicating the starting position for generating the sub-packet includes a fixed RV and an adaptive RV.
In the fixed RV, in order to obtain an average HARQ gain regardless of the variable length of the sub-packet in each HARQ transmission, the size N of the circular buffer is divided by the number V of starting position candidates, and V fixed positions are defined on the circular buffer at a gap of N/V therebetween.
In the case where the fixed RV is used, since adaptability for the variable length of the sub-packet deteriorates, an overlapping portion between sub-packets used for the HARQ transmission and re-transmission is increased. Since a portion of the codeword packet, which is not transmitted, is increased as the overlapping portion between the sub-packets is increased, a HARQ coding gain cannot be sufficiently obtained. If a larger number of starting position candidates are used in order to obtain the sufficient HARQ coding gain and reduce the gap between the starting positions of the fixed RV, a larger amount of control information should be used in order to transmit the RV information via the control channel.
FIG. 3 is a view showing an embodiment of the HARQ transmission in the case where the starting positions of four fixed RVs are used. In FIG. 3, it is assumed that a static channel is used and the length of the sub-packet used for each HARQ transmission is constant and is greater than N/3 or less than N/2.
In FIG. 3, a first transmission indicates a sub-packet used for initial transmission using the HARQ Method and other transmissions indicate sub-packets which are re-transmitted three times using the HARQ method. Meanwhile, in FIGS. 3 to 7, N indicates the size of the CB. FIG. 3 shows the case where the overall codeword packet stored in the circular buffer cannot be transmitted although the length of the sub-packet used for HARQ transmission is equal to or greater than N/3 and the initial transmission and two re-transmissions are performed. The adaptive RV is developed for solving the disadvantages of the fixed RV.
The adaptive RV is a method of defining the starting position of the RV using the variable length of the sub-packet used for each HARQ transmission. When the length of the codeword packet encoded at the mother code rate is N and the length of the sub-packet used for an ith HARQ transmission is Li, a jth starting position Si,j of the circular buffer for generating the sub-packet is expressed by Equation 1.Si,j=j*Li mod N  Equation 1
Equation 1 indicates the V starting position candidates for generating the sub-packet which will be transmitted in the ith HARQ transmission when j=0, 1, . . . , and V−1. In the ith HARQ transmission, since the transmitter has information on the starting positions and the ending positions of all the sub-packets used before the ith HARQ transmission, a sub-packet having a smallest overlapping portion between the sub-packet transmitted previously and the sub-packet which will be transmitted in the ith HARQ transmission or a sub-packet having a minimized gap with the position of the sub-packet transmitted previously may be selected and transmitted. It is possible to increase the HARQ coding gain by the selection of the sub-packet. FIG. 4 is a view showing a method of selecting and transmitting the sub-packet having a minimized gap with the position of the sub-packet transmitted previously.
In each transmission, the transmitter transmits the sub-packet via the data channel and transmits the RV information via the control channel. The RV information RVi=j indicates the agreed starting position Si,j obtained by Equation 1. The receiver can map the sub-packet transmitted via the data channel to the accurate position of the codeword packet using the RV information transmitted via the control channel.
FIG. 5 is a view showing an embodiment of the HARQ transmission using the starting positions of four adaptive RVs using Equation 1. In FIG. 5, it is assumed that a static channel is used and the length of the sub-packet used for each HARQ transmission is constant and is greater than N/3 or less than N/2, similar to FIG. 3.
Accordingly, the starting position of RVi=j obtained by Equation 1 always indicates the same starting position Si,j, regardless of i. Compared with FIG. 3, if the length of the sub-packet used for each HARQ transmission is identical and the length of the sub-packet is equal to or greater than N/3, it can be seen that the overall codeword packet stored in the circular buffer can be transmitted by three transmissions. Accordingly, it is possible to obtain a maximum coding gain by a smaller number of times of HARQ re-transmission.
In FIG. 5, the length of the sub-packet transmitted for each HARQ transmission is not changed. If the adaptive RV is used, since the gap between the starting position candidates used for all the HARQ transmissions is equal to the length of the sub-packet, incremental redundancy (IR) having the characteristics that the sub-packets used for all the HARQ transmissions are maximally orthogonal to each other can be realized.
Generally, in the initial transmission using the HARQ method, priority is given to the systematic bits of the codeword packet and the sub-packet including all the systematic bits is first transmitted.
In the conventional method of generating the sub-packet, since puncturing of the systematic bits for improving performance is not considered when the starting position of the sub-packet to be transmitted adaptively is decided with respect to a variable packet length, it is difficult to obtain a sufficient HARQ gain.